Such monitoring units are used e.g. for the monitoring of hazardous machine working zones. If an object moves into the monitored zone and thus into the light path between the light grid and the light receiver, an object detection signal is generated which results, for example, in the switching off of the machine or in the triggering of an acoustic or visual signal.
Not only articles are termed an “object” in the present text, but also and in particular body parts, e.g. of machine operators.
A monitoring unit is described in EP 0 875 873 B1 in which a spatially resolving receiver element is used, e.g. in the form of a photodiode matrix. The desired position of the light spot generated at the light receiver by the light transmitter is taught and stored in a teaching process at the location of use. During monitoring operation, the measured signal is compared with the stored desired signal to detect a deviation when e.g. an object is located in the light path and effects a shadowing.
Under unfavorable operating conditions, it can occur that an object in the monitored zone is “reflection bypassed” by reflections of the measured light at reflecting objects, e.g. in the marginal region of the monitored zone and that nevertheless a signal is measured at the position of the light receiver at which the shadowing of the measured light by the object would occur. To prevent this erroneous measurement, in the known solution, the whole area of the spatially resolving detected is read out, that is e.g. all the individual elements when a photodiode matrix is used. The risk of an erroneous detection due to reflection bypass can be reduced by a spatially resolved comparison of the total receiver surface with the expected signal.
In this respect, it is necessary with the known solution for the avoidance of such erroneous detections to read out the total receiver surface, that is e.g. all the photodiodes of the photodiode array of the receiver. This results in a high evaluation time.
DE 102 31 178 A1 describes an optoelectronic sensor in which the light of an individual light source is sent in the direction of a reflection element and moves from there to a light receiver. The received signals of individual receiver elements of a photodiode array are associated with different processing devices with the help of a switch matrix.
An optoelectronic security system having the features of the preamble of claim 1 is known from EP 0 875 873 B1.